Hemiacetal

Skeletal formula of a hemiacetal

Hemiacetals and hemiketals are compounds of the general formula R₁R'₁C(OH)OR₂, where R₂ is not hydrogen^[1][2]. These types of compounds are formed from carbonyl compounds and alcohols, i.e. hemiacetals from aldehydes, hemiketals from ketones. Thus, for hemiacetals, either R₁ or R'₁ must be a hydrogen, while for hemiketals, neither will be.

Formation of hemiacetals

Formation of hemiketals

Synthesis

In organic synthesis, hemiacetals can be prepared in a number of ways:

• Nucleophilic addition of an alcohol to a carbonyl group of an aldehyde
• Nucleophilic addition of an alcohol to a resonance stabilized hemiacetal cation
• Partial hydrolysis of an acetal

Reactions

Hemiacetals and hemiketals may be thought of intermediate between alcohols and aldehydes or ketones, and acetals and ketals:

-C=O + 2 ROH ⇌ -C(OH)(OR) + ROH ⇌ -C(OR)₂ + H₂O

A hemiacetal can react with an alcohol under acidic conditions to form an acetal, and can dissociate to form an aldehyde and an alcohol.

• hemiacetal + alcohol (excess) + acid (catalyst) ↔ acetal + water

An aldehyde dissolved in water exists in equilibrium with low concentrations of its hydrate, R-CH(OH)₂. Similarly, in excess alcohol, the aldehyde, its hemiacetal, and its acetal all exist in solution.

Hemiacetal results from addition of the alcohol's hydroxyl group to the carbon in the C=O bond. Acetals are products of substitution reactions catalyzed by acid. The presence of acid improves the leaving capacity of the hydroxyl group and enables its substitution with an alkoxyl group (-OR). The conversion of a hemiacetal to an acetal is an S_N1 reaction.

Ketones give hemiketals and ketals. These do not form as readily as hemiacetals and acetals. To increase yields of ketals or acetals, water formed during the reaction can be removed.

Cyclic hemiacetals and cyclic hemiketals in nature

Hemiacetals and hemiketals are generally unstable compounds. In some cases however, stable cyclic hemiacetals and hemiketals can be readily formed, especially when 5- and 6-membered rings are possible. Glucose and many other aldoses exist as cyclic hemiacetals whereas fructose and similar ketoses exist as cyclic hemiketals.

References

1. ^ International Union of Pure and Applied Chemistry. "hemiacetals". Compendium of Chemical Terminology Internet edition.
2. ^ International Union of Pure and Applied Chemistry. "hemiketals". Compendium of Chemical Terminology Internet edition.

See also

• Acetal
• Ketal